An observation device comprising an inner dome is known from European patent application EP 1136 964 A2 and from U.S. Pat. No. 3,819,856.
Cameras comprising at least an outer dome are generally used as observation cameras, in particular attached to a ceiling, with the outer dome functioning to protect the lens. Considering the nature of the use of such cameras, it is at least desirable or even necessary for such cameras to be vandal-proof. This implies that the outer dome must be resistant to a certain extent to impact with or by an object, such as a club or the like, so that the deformation that the outer dome undergoes as a result of such impact will be so small that the lens does not come into contact with the inner side of the outer dome and/or of the inner dome, for example, in which case the camera may require some form of repair as a consequence of the impact but continues to function as such.
In order to comply with this wish/requirement, the diameter of the outer dome is generally selected so large that there is a considerable spacing between the inner side of the outer dome and the lens. Thus the outer dome can deform to a large extent without the lens being touched. This solution to the problem as described above is contrary to the general wish/requirement to make the cameras small and compact so that they will be as inconspicuous as possible. After all, practice teaches that the less conspicuous a camera is, the less vandals will be inclined to test the camera's resistance to vandalism. In addition to that, thicker outer domes generally result in greater image distortion.
An alternative solution to increasing the resistance of cameras to vandalism is to user a thicker material for the outer dome. Apart from the fact that this leads to a relatively great increase of the manufacturing costs of such outer domes, it has become apparent that it may have a contrary effect on the impact-resistance, because the internal stresses that occur upon impact will be higher, which may lead to unacceptable failure of the outer dome.
The camera as described in U.S. Pat. No. 3,819,856 comprises an outer dome as well as an inner dome. The reason for using two domes for each camera as described in the aforesaid two documents is to provide a visual screen for the lens, so that it will be difficult for persons to detect the direction of the scanning beam and thus find out if they are in the camera's field of view. The outer dome in EP 1 136 964 A2 functions to provide a universal screen for different types of cameras.
From U.S. Pat. No. 4,920,326 there is known a surveillance camera comprising a substantially spherical housing. The housing substantially comprises an upper dome and a lower dome. The upper dome is formed of three layers, viz. an outer layer of a rubber-containing plastic alloy having good ballistic properties, an acrylic intermediate layer, and an aluminum inner layer. The lower dome comprises the same three layers, with two transparent polycarbonate layers additionally being present on the inner side and on the outer side. Pop rivets are provided to keep the transparent polycarbonate layers and the three layers present therebetween together. On the outside, the lower dome is finally provided with a methyl acrylate cover. The scanning beam passes through the two transparent polycarbonate layers and through the methyl acrylate layer, and air is present between the three layers. A very important drawback of the known observation device is the constructional complexity thereof, as a result of which the manufacture of the various parts and the assembly thereof will be time-consuming and costly. Another important drawback is the use of the large number of dome layers, resulting in a loss of light with the attendant risk of significant refraction of the light. Moreover, it is difficult to gain access to the interior of the observation device.